Water Rocket Forum

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A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel (the engine of the rocket) is constructed from thin plastic or other non metallic materials (usually a used plastic soft drink bottle) weighing 1,500 grams or less. The water is forced out by compressed air. It is an example of Newton's third law of motion.

I have been reading a bit about what is legal for use as pressure chamber in rockets for the competitions and i am a bit in the dark.
Seeing as the availability af suitable bottles or other tubings for pressure vessels are extremely limited and hard to come by, let alone expensive in Denmark , i was planning on making the main pressure chamber out of the inner ring of a bicycle tire cut up to remove the valve.

This however seems as though it would directly violate the rules thus making the rocket a non legitimate competition rocket as the inner tube even though cut and straightened out was originally designed for pressure.

So i would very much like to get this clarified before i decide to charge ahead with this particular design instead of my other designs.

I have been reading a bit about what is legal for use as pressure chamber in rockets for the competitions and i am a bit in the dark.
Seeing as the availability af suitable bottles or other tubings for pressure vessels are extremely limited and hard to come by, let alone expensive in Denmark , i was planning on making the main pressure chamber out of the inner ring of a bicycle tire cut up to remove the valve.

This however seems as though it would directly violate the rules thus making the rocket a non legitimate competition rocket as the inner tube even though cut and straightened out was originally designed for pressure.

So i would very much like to get this clarified before i decide to charge ahead with this particular design instead of my other designs.

A bicycle inner tube that is modified would be allowed (for competitions not requiring the use of an unmodified soft drink bottle) but would not alone make a good rocket. You must be planning on using it as a bladder inside some other container or reinforcing it as it would be too flexible. Which competition are you planning on entering? (some of the competitions do not allow any modifications to soft drink bottles).

A word of advice: Pay attention to your weight (1,500 gram limit) and don't use any metal when constructing your pressure vessel (common mistakes that have been made in the past).

Lisa Walker,Forum Administrator. The Water Rocket Achievement World Record Association

I was personally going to just go for the A class, highest altitude, if by some dumb luck i manage to find suitable bottles in my country i will probably also try for reinforced bottles. The biggest problem is the only bottles i can find that have straight sides are half liter noncarbonated drink bottles whose pressure limits i don't hold high hopes for.

I had also planned an using some external tubing to contain the bladder, although i have yet to decide on what material as home supply depots here don't carry the same range of usable materials. I have considered PVC piping however it is rather heavy and thus wouldn't be of much use. I can't even use PFC as i have yet to find a supplier inside my country.

In any case it will probably take a long time before i start competing as i also need to get a hold of recording equipment and altimeter.

Okay i have just tested some bottles i had lying around from some noncarbonated vitamin drinks, and they had the proper shape to use. Subsequent pressure testing has confirmed that they can easily withstand pressures of up to 160 psi with no deformation.
As such i will not be needing to make a bladder out of rubber tubing and thus my question here is no longer relevant.
Sorry for the trouble of asking before checking my apartment thouroughly, and thanks for the answer given to my question.

mazuzuri wrote:Okay i have just tested some bottles i had lying around from some noncarbonated vitamin drinks, and they had the proper shape to use. Subsequent pressure testing has confirmed that they can easily withstand pressures of up to 160 psi with no deformation.
As such i will not be needing to make a bladder out of rubber tubing and thus my question here is no longer relevant.
Sorry for the trouble of asking before checking my apartment thouroughly, and thanks for the answer given to my question.

Those must be strong bottles to hold 160PSI without deforming. Are they thick wall bottles? The bottles we have available have thin walls and they will swell up even with relatively low pressures. Thankfully, they just swell up and still hold the pressure until you make it so great they burst.

Yes they are relatively thick walled, i have measured them to a thickness of 0.62mm.
This is almost 2.5 times thicker than the standard thin walled bottles that i have measured to 0.25mm.

However these thicker walled bottles only come in 0.5liter size.
On top of that they are unusable for slip joint splicing, as they do not crimp noticably even in boiling water, apart from that the edge curls outwards even when a cardboard ring is used to control the curling.

I have however devised a rocket design that i think they would be perfect for seeing as this rocket won't be a high performer but more of a childhood dream come true.
I will be making a log of that at some point after getting a reliable deployment mechanism going.

Sorry for doubble posting, i thought that you might be interested in a picture of the actual bottletype that i mentioned besides a standard thinwalled bottle that is more or less the standard curly designer bottle available to me.

As you can see the thick walled has several advantages, the most notable being it is completely smooth and with a halfsphere bottle neck. The bottom also has far smaller indentations and is also between 2-3 times as thick as the rest of the bottle meaning its between 1-1.5 mm thick.